This invention relates to the hydrogenation of unsaturated groups in the presence of transition metal catalysts. More particularly, this invention relates to the reaction of hydrogen with unsaturated hydrocarbons in the presence of a transition metal catalyst containing a cyclometallated group to provide the corresponding saturated hydrocarbon. The use of transition metal catalysts for olefin hydrogenation is well-known in the art as indicated by Kirk-Othmer in Encyclopedia of Chemical Technology, 6, (1978), pp. 583-584. James discusses a series of transition metal catalysts within Advancements in Organometallic Chemistry, 17 (1979) 319. In addition, Kirk-Othmer describes transition metal catalyst suitable for hydrogenation in the Encyclopedia of Chemical Technology (1978) at volume 6, p. 793 and volume 4, p. 842, which include nickel, cobalt, platinum, palladium, chromium, zinc, rhodium and molybdenum. Complexes of these transition metals are utilized to provide catalysis of the hydrogenation reaction within a homogeneous system. Many of these transition metal complexes are sensitive to moisture and air and lose their activity in the homogeneous system very quickly. It is desirable to obtain a transition metal complex which exhibits greater stability within the reaction medium and is less susceptible to oxidation in the presence of air and moisture.
Cyclometallated transition metal complexes have been described by Dehand and Pfeffer in Coordination Chemistry Reviews, 18 (1976) 327-352 and Michael Bruce, in Angew. Chem. Int. Ed. Eng., 16 (1977) 73-86, which are incorporated herein by reference. These references discuss various species of cyclometallated complexes, their syntheses, their physical properties and some chemical reactions of the ring structures of the cyclometallated complexes. The use of the cyclometallated complexes as hydrogenation catalysts has heretofore never been suggested. The cyclometallated complexes have never been employed in a catalytic system.